The present invention relates to a molded article based on PZT (Pb(Zr.sub.x,Ti.sub.y)O.sub.3, Lead zirconate-lead titanate, with x+y=1), and a method and intermediate product for making it. More particularly, the invention relates to a method of making PZT endless fibers.
Lead zirconate (PbZrO.sub.3) and lead titanate (PbTiO.sub.3) form a continuous series of mixed crystals having a Perovskite-similar structure. Polycrystalline lead zirconate-titanate (Pb(Zr,Ti)O.sub.3, PZT) is an important ferroelectric material, for which numerous electronic applications exist because of its attractive dielectric, piezoelectric and electro-optical properties.
Many application-relevant ferroelectric properties attain their optimum values in the vicinity of the morphotropic phase boundary between the zirconate-rich rhombohedral modification and the titanate-rich tetragonal modification.
Besides variation of properties by changing the Zr/Ti ratio, the properties of this material can be varied by changing the crystallite size, by doping and by partial or complete substitution of individual components.
In doping small quantities of foreign ions with varying charges are added(usually less than 10%), which modify the crystal structure. A great variety of metal ions can be used as doping agents, e.g. La.sup.3+, Nd.sup.3+, Sb.sup.3+, Bi.sup.3+, Th.sup.3+, Nb.sup.5+, Sb.sup.5+, W.sup.6+, Na.sup.+, Fe.sup.3+, Fe.sup.2+, Co.sup.3+, Co.sup.2+, etc. The doping with lanthanum improves the optical properties significantly(transparency, double refraction), so that PLZT-materials above all are of significance for electro-optical applications.
The structure and thus the properties of these materials change also when the Pb, Zr or Ti ions present are replaced by ions having a different ionic radius. Pb.sup.2+ is frequently replaced by an alkaline earth ion (Mg.sup.2+, Ca.sup.2+, Sr.sup.2+, Ba.sup.2+) and Ti.sup.4+, Zr.sup.4+ may be replaced by, e.g., Sn.sup.4+ and Hf.sup.4+.
For innovative applications of ferroelectric materials in composite materials PZT-long fibers, which can perform sensor or also actuator functions, are interesting materials. The manufacture of PZT-fibers has been the subject of research for several years. There are three different methods:
Precursor Methods (by a Sol-Gel Process). A precursor sol is obtained by addition of water to a methanol solution of Pb(OAc).sub.2 .multidot.3H.sub.2 O, Zr(OPr.sup.n).sub.4, Ti(OPr.sup.i).sub.4, from which a viscous fluid is obtained after removal of volatile components, from which a 1 m long fiber can be drawn in manual experiments. Polyvinyl-pyrrolidone is worked into the spinning mass to improve the fiber drawing properties and PZT-powder is worked into it to make a thick, porous fiber(Mat. Res. Soc. Symp. Proc., 271, pp. 517-523 (1992)). This process is disadvantageous since only fibers of insufficient length can be made by it. An additional disadvantage is that the process operates using a solvent and in connection with a drying process for removal of the solvent so that the solvent must be disposed of.
Extrusion Methods. Commercial PZT-powder (e.g. PZT 501A, Ultrasonic Powders Inc.) is worked into a 20% by weight polyvinylalcohol solution, which is extruded and the extrudate sintered. If it is sintered at low temperatures, one does not obtain a compact fiber, and, if it is sintered at high temperatures, a heterogeneous mixture is produced since powdery PZT is used. An additional disadvantage of this method is that the fiber thickness is influenced by the bulk density and that thick fibers result. Fine-grained powder, which can be made only by expensive methods, is required to make thinner fibers. Furthermore the fibers must be sintered which requires high temperatures. Fault formation is possible because of the sintering process and fault locations result in the structure which leads to a reduction of the properties.
The replication methods. Carbon fiber mats are soaked in a PZT precursor Sol. After removal of the carbon fibers by combustion at 600.degree. C. a PZT hollow fiber framework or grid results, which is sintered at 1285.degree. C. A spinning process is not involved in the formation of the hollow fiber framework, but instead a dipping process, i.e., a coating process, is used, which is difficult to perform with individual fibers. Furthermore these methods are used at high temperatures, which make the process very cost intensive. Moreover the production of thick fibers is impossible by this coating process. Furthermore the operation of this process is poor and allows only slight variation of the thickness of the resulting fibers.
Another process for making PZT-fibers is described in U.S. Pat. No. 5,072,035. A mixture of Zr(OPr.sup.n).sub.4 and Ti(OPr.sup.i).sub.4 is heated for two hours under reflux in a dry nitrogen atmosphere, reacted with one or more lead organometallic compounds, and heated for at least four additional hours. The liquid mixture is concentrated at 80.degree. C. until a yellow-brown, viscous, resinous mass is produced, from which fibers are drawn. This process is cumbersome, involves considerable effort and is costly because of the requirement of a lengthy reflux time and exclusion of water excluded(under a dry nitrogen atmosphere).